To create an expression in MEL, you would do this:

expression -s "ls.rotateY = time;" -o expression1 -ae 1 -uc all ;
// Result: expression1 //

I actually did not code that, I did it in the expression editor and just copied it out of the script editor. . . But it is handy to know how to write the code. If you want to edit an expression after it has already been created, you would do this:

expression -e -s "ls.rotateY = time * 3;" -o expression1 -ae 1 -uc all expression1;
// Result: expression1 //

Note the extra -e in the edit expression code. You also must name the expression which you wish to edit! (expression1).

The usefullness in this coding is very well demonstrated in the Walkerman program as part of the code that controls the ability to toggle the hand control so you can either control all four fingers at once or one finger at a time . . . .